Evolve shellder to be a prestiger or attacker?

I was never lucky with lapras, so I have two cloyster at 13xx (hatched shellder, no powerup) and two cloyster at 20xx. Lapras has a much higher max CP and HP. Overall, a lapras will kill more dragons than a cloyster, but it will need more potions afterwards.

Honestly, cloyster can be either. Mine take out 1.5 3k dragons. It all depends on candy, moves, and level. If your cloyster gets HP, this question is moot. Dont both powering up. If it is a lower level (21 or under) and you have the lapras candy, you may be better served powering up a lapras as an attacker.

Additionally, piloswine is new and designed to hit dragonite as well.

Lastly, if you just have enough candy for cloyster, then what are you asking? Do you have candy to power up the cloyster too?

Tl;dr I don't think you have the candy to really power him up and you seem to have other options (power up lapras, get piloswine), so I would evolve a cloyster and keep him at that level, even if it gets double ice moves.

You probably don't need Cloyster considering your current roster. But if you are missing it from your Pokedex then why is this even a question?

I use it for both. I have three I use (one at 1400, one at 1700, and one at 2250) depending on the situation. They can all take down Dragonites quickly with limited potion use. If you only have one, I would focus on a prestiger unless you have enough candies to power it up beyond L30 or don't have another high CP ice attacker. Unfortunately, it's value will depend greatly on whether you get Avalanche.